1. Field of the Invention
This invention relates to a soldering flux and more particularly to a water-soluble soldering flux in which a residue of the flux remaining after soldering can be readily removed by washing with warm or cold water.
2. Prior Art
A soldering flux is used in soldering to chemically remove oxide films from the metal surface to be soldered and from the molten solder surface, thereby exposing solderable metal surfaces. It is therefore indispensable in all soldering processes.
A conventional flux used in soldering of electronic parts usually comprises rosin as the main activator. Such a rosin-based flux has good solderability, is non-corrosive and non-toxic, and the flux residue remaining after soldering has good electrical insulation properties. However, it is often necessary to wash off the residue of a rosin-based flux remaining after soldering, particularly when the flux is used to solder electronic parts of a precision electronic device such as a communication device or large-scale computer or of an important safety or maintenance device for an automobile, for example. This is because rosin is thermoplastic in nature and it is non-reactive at room temperature but is activated at elevated temperatures. Therefore, if any residue of the flux is left on or around a soldered part of a device, the insulating properties of the residual flux may be degraded at elevated temperatures, and as a result the device may operate improperly. For this reason, when a rosin-based flux is used in soldering, the flux residue remaining after soldering is completely removed in most cases by washing. Fluorinated or chlorinated organic solvents have conventionally been used to wash off the flux residue since they have a high dissolving power for rosin.
Nowadays, however, the use of fluorinated or chlorinated organic solvents is strictly regulated since these solvents tend to destroy the ozone layer surrounding the earth, thereby increasing the amount of ultraviolet rays, which are harmful to the human body, reaching the earth through the ozone layer. Another environmental problem involved in the use of such solvents is contamination of underground water by these solvents some of which are toxic.
Under these circumstances, increased attention is being directed to water-soluble soldering fluxes, which have the advantage that any flux residue remaining after soldering can be washed off with warm or cold water without use of a harmful organic solvent.
Several types of water-soluble soldering fluxes are known. They comprise as an activator an inorganic salt such as zinc chloride or ammonium chloride dissolved in glycerol or vaseline, or a hydrohalide salt of an amine dissolved in a water-miscible organic solvent.
However, these prior-art water-soluble soldering fluxes have the disadvantage that removal of flux residue deposited around the soldered areas by washing with water after soldering is rather difficult and labor- and time-consuming.
While it is desirable that any flux residue be completely removed after soldering, complete removal of flux residue deposited around soldered electronic parts is, in practice, very difficult. Even though flux residues cannot be completely removed by washing, the small amount of flux residues remaining around soldered areas will not create any serious problem if they do not decrease the insulation resistance of the soldered areas. However, the water-soluble soldering fluxes known in the art are hygroscopic, so even if only a slight amount of a residue of such a flux is left after soldering, it will decrease the insulation resistance of the soldered areas.